Carbon nanotubes—graphene hybrid structures for separator free silicon—sulfur batteries

ABSTRACT

Provided herein are electrochemical systems and related methods of making and using electrochemical systems. Electrochemical systems of the invention implement novel cell geometries and composite carbon nanomaterials based design strategies useful for achieving enhanced electrical power source performance, particularly high specific energies, useful discharge rate capabilities and good cycle life. Electrochemical systems of the invention are versatile and include secondary lithium ion cells, such as silicon-sulfur lithium ion batteries, useful for a range of important applications including use in portable electronic devices. Electrochemical cells of the present invention also exhibit enhanced safety and stability relative to conventional state of the art lithium ion secondary batteries by using prelithiated active materials to eliminate the use of metallic lithium and incorporating carbon nanotube and/or graphene, composite electrode structures to manage residual stress and mechanical strain arising from expansion and contraction of active materials during charge and discharge.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent App. No. 61/842,511, filed Jul. 3, 2013, and U.S. ProvisionalPatent App. No. 61/939,996, filed Feb. 14, 2014, each of which is herebyincorporated by reference to the extent not inconsistent herewith.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF INVENTION

Over the past few decades revolutionary advances have been made inelectrochemical storage and conversion devices significantly expandingthe capabilities and applications of these systems. Current state of theart electrochemical storage and conversion devices implement celldesigns specifically engineered to achieve performance attributesenabling requirements and operating conditions supporting diverseapplications including portable electronics, transportation, energy,lighting, sensing and communications. Despite the development andwidespread adoption of this suite of advanced electrochemical storageand conversion systems, significant pressure continues to stimulateresearch to expand the functionality of these systems, thereby enablingan even wider range of device applications. The rapid and ongoing growthin the demand for high power portable electronic products, for example,has created enormous interest in developing safer, light weightsecondary batteries capable of achieving higher energy densities andcycling performance.

Many recent advances in electrochemical storage and conversiontechnology are directly attributable to discovery of new materials forkey battery components. Lithium battery technology, for example,continues to rapidly develop, at least in part, due to the discovery ofnovel electrode and electrolyte materials for these systems. From thepioneering discovery and optimization of intercalation host materialsfor positive electrodes, such as fluorinated carbon materials andnanostructured transition metal oxides, to the development of highperformance non-aqueous electrolytes, the implementation of novelmaterials strategies for lithium battery systems have revolutionizedtheir design and performance capabilities. Furthermore, development ofintercalation host materials for negative electrodes in these systemshas led to the discovery and commercial implementation of lithium ionsecondary batteries exhibiting high capacity, good stability and usefulcycle life. As a result of these advances, lithium based batterytechnology is currently widely adopted for use in a significant range ofapplications including primary and secondary electrochemical cells forportable electronic systems.

Commercial primary lithium battery systems typically utilize a lithiummetal negative electrode for generating lithium ions which aretransported during discharge through a liquid phase or solid phaseelectrolyte and undergo intercalation reaction at a positive electrodecomprising an intercalation host material. Dual intercalation lithiumion secondary batteries have also been developed, wherein lithium metalis replaced with a lithium ion intercalation host material for thenegative electrode, such as carbons (e.g., graphite, coke, etc.), metaloxides, metal nitrides and metal phosphides. Simultaneous lithium ioninsertion and de-insertion reactions allow lithium ions to migratebetween positive and negative intercalation electrodes during dischargeand charging. Incorporation of a lithium ion intercalation host materialfor the negative electrode also has the significant advantage ofavoiding the use of metallic lithium which is susceptible to safetyissues upon cycling attributable to the highly reactive nature andnon-epitaxial deposition properties of metallic lithium.

The element lithium has a unique combination of properties that make itattractive for use in high performance electrochemical cells. First, itis the lightest metal in the periodic table having an atomic mass of6.94 AMU. Second, lithium has a very low electrochemicaloxidation/reduction potential (i.e., −3.045 V vs. NHE (normal hydrogenreference electrode). This unique combination of properties enableslithium based electrochemical cells to achieve high specific capacities.Advances in materials strategies and electrochemical cell designs forlithium battery technology have realized electrochemical cells capableof providing useful device performance including: (i) high cell voltages(e.g. up to about 3.8 V), (ii) substantially constant (e.g., flat)discharge profiles, (iii) long shelf-life (e.g., up to 10 years), and(iv) compatibility with a range of operating temperatures (e.g., −20 to60 degrees Celsius). As a result of these beneficial characteristics,lithium and lithium ion batteries are currently the most widely adoptedpower sources in portable electronic devices, such as cellulartelephones and portable computers. The following references are directedgenerally to lithium and lithium ion battery systems which are herebyincorporated by reference in their entireties: U.S. Pat. Nos. 4,959,281;5,451,477; 5,510,212; 6,852,446; 6,306,540; and 6,489,055; and “LithiumBatteries Science and Technology” edited by Gholam-Abbas Nazri andGianfranceo Pistoia, Kluer Academic Publishers, 2004.

Despite these advances, significant challenges remain to be addressedfor the continued development of lithium ion batteries including issuesrelating to the cost, electrochemical performance and safety of thesesystems. Advances in cathode active materials, such as LiMn₂O₄, LiCoO₂and LiFePO₄, have accessed improved device performance. [See, e.g., U.S.Pat. Nos. 5,763,120; 5,538,814; 8,586,242; 6,680,143 and 8,748,084].Such advanced cathode active materials, however, are still limited inthe overall energy densities achievable and also bring into playsignificant issues with overall conductivity, cycling performance andtoxicity for some of the materials. High capacity anode activematerials, such as nanostructured Si, Sb, Sn, Ge and alloys thereof,access higher specific capacities and allow for elimination of the useof metallic lithium to avoid problems associated with dendritic growth.[See, e.g., US Publication 2013/0252101 and U.S. Pat. No. 8,697,284].Such advanced anode active materials, however, are susceptible to largechanges in volume upon charging and discharge which can cause structuraldegradation upon cycling resulting in capacity fading, poor cycle life,lower system efficiency and increased internal resistance. Moreover,many commercial lithium ion systems implementing advanced cathode andanode active materials typically exhibit actual specific energies 4 to12 times smaller that the specific energy of the electrodes due to thesignificant weight of other battery components such as separators,electrolytes, current collectors, connectors and packaging components.

As will be clear from the foregoing, there exists a need in the art forsecondary electrochemical cells for a range of important deviceapplications including the rapidly increasing demand for highperformance portable electronics. Specifically, secondaryelectrochemical cells are needed that are capable of providing usefulcell voltages, specific capacities and cycle life, while at the sametime exhibiting good stability and safety. A need exists for alternativecell geometries and intercalation based electrochemical cells thateliminate or mitigate safety issues inherent to the use of lithium inlithium ion battery systems.

SUMMARY OF THE INVENTION

Provided herein are electrochemical systems and related methods ofmaking and using electrochemical systems. Electrochemical systems of theinvention implement novel cell geometries and composite carbonnanomaterials-based design strategies useful for achieving enhancedelectrical power source performance, particularly high specificenergies, useful discharge rate capabilities and good cycle life.Electrochemical systems of the invention are versatile and includesecondary lithium ion cells, such as silicon-sulfur lithium ionbatteries, useful for a range of important applications including use inportable electronic devices. Electrochemical cells of the presentinvention also exhibit enhanced safety and stability relative toconventional state of the art lithium ion secondary batteries by usingprelithiated active materials to eliminate the use of metallic lithiumand incorporating carbon nanotube and/or graphene, composite electrodestructures to manage residual stress and mechanical strain arising fromexpansion and contraction of active materials during charge anddischarge.

In an embodiment, electrochemical cells of the invention integratecarbon nanotube templated composite structures for positive and negativeelectrodes to achieve overall cell geometries that eliminate the needfor a conventional separator component, thereby increasing specificcapacity. Moreover, carbon nanotube templating approaches of theinvention are also complementary for incorporation of high performanceactive materials, such silicon and sulfur, in useful form factors fornegative and positive electrodes, for example, by providing highinternal conductivity and an effective means of reducing residual stressresulting from expansion and contraction of materials during charge anddischarge cycles. In addition, electrochemical cells of some embodimentsintegrate graphene enclosure structures for active materials to reduce,or outright prevent, transport of certain reaction products that cancause degradation of the electrolyte, such as polysulfides generatedduring charge and discharge using a sulfur active material for thepositive electrode. In addition, electrochemical cells of someembodiments integrate composite electrode structures having acombination of silicon and sulfur active materials, thereby supportinghigh electrochemical performance while avoiding use of toxic and/orunstable materials.

In an aspect, the invention provides an electrochemical cell comprising:(i) a negative electrode comprising a first assembly of carbon nanotubessupporting a silicon active material; (ii) a positive electrodecomprising a second assembly of carbon nanotubes supporting a sulfuractive material; and (iii) an electrolyte provided between the positiveelectrode and the negative electrode; the electrolyte capable ofconducting charge carriers; wherein the first assembly of carbonnanotubes and the second assembly of carbon nanotubes are physicallyseparated from each other and are supported by a common surface. In anembodiment, for example, the electrochemical cell is a lithium ionbattery, for example, wherein the charge carriers are Li⁺ ions, andwherein the positive electrode and the negative electrode are capable ofaccommodating the Li⁺ ions during charge or discharge of theelectrochemical cell. In an embodiment, for example, the silicon activematerial, the sulfur active material or both are prelithiated materials,for example, active materials that have been electrochemicallyprelithiated. Incorporation of prelithiated active materials in thepresent electrochemical cells is a strategy useful to avoid safety andperformance issues arising from formation of dendrites of metalliclithium during charge and discharge cycling.

Composite electrodes comprising active materials supported by carbonnanotube assemblies enable a wide range of cell geometries providingbenefits for electrochemical performance and safety. In an embodiment,for example, the nanotube assembly-based composite structures of theinvention allow electrodes to be physically separated from each otherwithout the use of a conventional separator component, such as apermeable membrane, thereby enabling electrochemical cells exhibitinghigh specific capacities. In an embodiment, for example, the nanotubeassembly-based composite structures of the invention allow electrodes tobe patterned in a wide range of spacing filling geometries supportinggood discharge rate performance such as strip geometries, interleavedgeometries and spiral geometries. In an embodiment, for example, thenanotube assembly-based composite structures of the invention allowelectrodes to be patterned in a geometry wherein the typical load on theelectrochemical cell does not generate force acting upon the positive ornegative electrode in the direction of the opposing electrode. In anembodiment, the first assembly of carbon nanotubes and the secondassembly of carbon nanotubes are physically separated from each other byat least 10 μm, optionally at least 100 μm, for example, as provided byone or more void regions between positive and negative electrodesoccupied by electrolyte.

The invention includes cell geometries wherein positive and negativeelectrodes are supported by a common substrate, such as a polymer,inorganic, ceramic, metallic or composite substrate. In someembodiments, for example, the electrochemical cell further comprises asubstrate, wherein the common surface supporting the first assembly ofcarbon nanotubes and the second assembly of carbon nanotubes is anexternal surface of the substrate. The external surface supportingpositive and negative electrodes can be in a planar or nonplanarconfiguration, such as a cylindrical, folded, bent or woundconfiguration. In an embodiment, for example, the first assembly ofcarbon nanotubes and the second assembly of carbon nanotubes areprovided on the external surface of the substrate or on one or moreintermediate structures provided between the first assembly of carbonnanotubes and/or the second assembly of carbon nanotubes and theexternal surface of the substrate.

In an embodiment, the first and second carbon nanotube assemblies areindependently in electrical contact, and optionally in physical contact,with one or more current collector structures supported by, andoptionally in physical contact with, the substrate. In an embodiment,for example, the first assembly of carbon nanotubes is provided on afirst current collector supported by the external surface of thesubstrate and the second assembly of carbon nanotubes is provided on asecond current collector supported by the external surface of thesubstrate. In an embodiment, for example, the electrochemical cellfurther comprises a first graphene electrical interconnect and a secondgraphene interconnect, wherein the first graphene electricalinterconnect is provided between the first assembly of carbon nanotubesand the first current collector; and wherein the second grapheneelectrical interconnect is provided between the second assembly ofcarbon nanotubes and the second current collector. Electrochemical cellsof the invention may comprise a range of current collectors, such ascarbon current collectors, metallic current collectors, particulatecurrent collectors, etc.

Electrodes of the present cells may be provided in a wide range of spacefiling geometries, for example, accessed by a range of patterningtechniques for carbon nanotube assemblies, including patterning carbonnanotube growth catalysts on substrates and/or current collectors usinglithographic and/or liquid phase deposition approaches. In anembodiment, for example, the first assembly of carbon nanotubes isprovided as one or more first strips supported by the substrate and thesecond assembly of carbon nanotubes is provided as one or more secondstrips supported by the substrate. Using nanotube pattern techniquesknown in the art, the physical dimensions (e.g. thickness, width,length, etc.), physical properties (e.g., nanotube surfaceconcentration, density, intertube spacing, etc.), position and shape ofstrips of nanotubes comprising first and second assemblies can beaccurately defined, including processing involving first spatiallypatterning a substrate, interconnect and/or current collector withnanotube growth catalyst followed by exposure to gas phase or liquidphase precursors resulting in growth of nanotube arrays or networkslocalized to regions of the substrate spatially defined by the patternof catalyst.

In certain embodiments, such first and second set of strips define thephysical dimensions, shape, separation and overall form factors of thepositive and negative electrodes. In an embodiment, for example, thefirst strips are separated from the second strips by at least 10 μm,optionally for some applications at least 50 μm, and optionally for someapplications at least 100 μm. In certain embodiments, the first stripsand the second strips independently have a geometry characterized bywidths selected from the range of 10 μm to 1 mm, optionally for someapplications 100 μm to 1 mm, and lengths selected from the range of 30μm to 3 mm, optionally for some applications 300 μm to 3 mm. In anembodiment, for example, the first strips and the second strips arearranged in a space filling geometry. In an embodiment, for example, thefirst strips and the second strips are arranged in a parallel geometry,an interleaved geometry, a coiled geometry, a nested geometry or aspiral geometry.

Carbon nanotube assemblies impart beneficial mechanical, electronic andchemical properties of positive and negative electrodes of the presentelectrochemical cells. A variety of carbon nanotube compositions andgeometries are useful in the nanotube assemblies of the invention. In anembodiment, for example, the carbon nanotubes of the first assembly andthe second assembly comprise single walled carbon nanotubes, multiwalledcarbon nanotubes or a mixture of both single walled carbon nanotubes andmultiwalled carbon nanotubes. Use of multiwalled carbon nanotubes innanotube assemblies for positive and negative electrodes is preferredfor certain embodiments given their high conductivity and largerphysical dimensions relative to single walled carbon nanotubes. Thus,the invention includes composite electrode structures comprising carbonnanotubes that are exclusively multiwalled nanotubes. In an embodiment,for example, the carbon nanotubes of the first assembly and the secondassembly comprise primarily, and optionally solely, metallic carbonnanotubes. In an embodiment, for example, the carbon nanotubes of thefirst assembly and the second assembly are independently characterizedby radial dimensions (e.g., diameter) selected over the range of 5 nm to100 nm, optionally for some applications 20 nm to 100 nm, and lengthdimensions selected over the range of 10 μm to 5 mm, optionally for someapplications 100 μm to 5 mm. In an embodiment, for example, the carbonnanotubes of the first assembly and the second assembly areindependently characterized by radial dimensions (e.g., diameter)greater or equal to 20 nm and length dimensions greater or equal to 1000μm. In an embodiment, for example, the carbon nanotubes of the firstassembly and the second assembly are independently characterized by anaverage surface concentration greater than or equal to 25 nanotubes μm⁻²and optionally greater than or equal to 100 nanotubes μm⁻².

Nanotube assemblies of the invention include carbon nanotube arrays,including arrays comprising spatially aligned carbon nanotubes, and/or acarbon nanotube networks, including random nanotube networks. In anembodiment, for example, the carbon nanotubes of the first assemblycomprise a first array of vertically aligned carbon nanotubes and thecarbon nanotubes of the second assembly comprise a second array ofvertically aligned carbon nanotubes. A used herein, vertically alignedrefers to alignment of nanotubes in an assembly along axes intersecting,and optionally orthogonal, to a supporting substrate or currentcollector. In an embodiment, for example, the vertically aligned carbonnanotubes of the first array and the second array extend in one or moredirections away from the common surface. In an embodiment, for example,the vertically aligned carbon nanotubes of the first array and thesecond array extend in a common direction away from the common surface.In an embodiment, for example, the vertically aligned carbon nanotubesof the first array and the second array are independently characterizedby an average interspacing between adjacent nanotubes selected over therange of 10 nm to 200 nm. In some embodiments, the nanotube assembliesare characterized by a configuration having a plurality of nanotubecrossings or a configuration characterized by carbon nanotubes in asubstantially parallel orientation (e.g., deviations from absoluteparallelism of less than 20%).

In an embodiment, for example, the carbon nanotubes of the first andsecond assemblies provides a mechanical scaffold to manage or minimizeforces acting on the electrodes that can cause degradation, loss ofconductivity and/or mechanical failure. In an embodiment, for example,the carbon nanotubes of the first assembly provide a mechanical scaffoldcapable of accommodating stress resulting from expansion of the siliconactive material during charging or discharge of the electrochemical cellso as to allow a reversible change in volume of the negative electrodegreater than or equal to 200% without mechanical failure. In anembodiment, for example, the carbon nanotubes of the second assemblyprovide a mechanical scaffold capable of accommodating stress resultingfrom expansion of the sulfur active material during charging ordischarge of the electrochemical cell so as to allow a reversible changein volume of the positive electrode greater than or equal to 200%without mechanical failure. This aspect of the invention is particularlyuseful for providing good cycle life for composite negative electrodeshaving a silicon active material, for example, by avoiding fracture,cracking and disconnection from the current collector duringcharge-discharge cycling.

In an embodiment, for example, the carbon nanotubes of the first andsecond assemblies provide an effective means of increasing the overallelectrical conductivity of the positive and/or negative electrodes. Inan embodiment, for example, the carbon nanotubes of the first assemblyand the carbon nanotubes of the second assembly independently increasethe overall conductivity of the positive electrode and the negativeelectrode by a factor greater than or equal to 10 and optionally forsome embodiments, greater than or equal to 100. For example, carbonnanotubes are typically more than 5 and 19 orders of magnitude moreelectrically conductive than silicon and sulfur, respectively. Forgeometries wherein carbon nanotubes are 0.1 vol % of the bulk electrode,an improvement in conductivity of about 100 times is achievable usingthe nanocomposite electrode structures of the invention. This aspect ofthe invention is particularly useful for providing useful conductivitiesfor composite positive electrodes having a sulfur active material highresistance.

The electrochemical cells of the invention are compatible with a rangeof active materials for positive and negative electrodes including highperformance, stable and nontoxic materials. In an embodiment, forexample, the silicon active material comprises a silicon-containingmaterial, such as elemental silicon or an alloy thereof. In anembodiment, for example the sulfur active material comprises asulfur-containing material, such as elemental sulfur. Significantadvantages of embodiments of the invention including silicon and sulfuractive materials include high specific capacity, low cost, and avoidanceof toxic materials such a conventional transition metal-metal cathodematerials.

In an embodiment, for example, the silicon active material and thesulfur active material independently comprises a single crystallinematerial, a polycrystalline material or amorphous material. In anembodiment, for example, the silicon active material and sulfur activematerial are provided in electrical contact, and optionally physicalcontact, with the carbon nanotubes of the first and second assemblies.In an embodiment, for example, the silicon active material is providedon the carbon nanotubes of the first assembly and/or the sulfur activematerial is provided on the carbon nanotubes of the second assemblyindependently by a process selected from the group consisting physicalvapor deposition, chemical vapor deposition, electrodeposition,sputtering, solution casting, liquid infusion and liquid deposition. Inan embodiment, for example, the silicon active material at leastpartially coats, and optionally fully coats, the carbon nanotubes of thefirst assembly, the sulfur active material at least partially coats thecarbon nanotubes of the second assembly or wherein both the siliconactive material at least partially coats, and optionally fully coats,the carbon nanotubes of the first assembly and the sulfur activematerial at least partially coats, and optionally fully coats, thecarbon nanotubes of the second assembly. In an embodiment, for example,the silicon active material provides a coating on at least a portion ofthe carbon nanotubes of the first assembly having a thickness greaterthan or equal to 0.1 μm. In an embodiment, for example, the sulfuractive material provides a coating on at least a portion of the carbonnanotubes of the first assembly having a thickness greater than or equalto 0.1 μm. In an embodiment, for example, positive and negativeelectrodes independently have overall thicknesses (i.e., activematerial+carbon nanotubes) selected from the range of 10 μm to 1 mm,optionally for some applications selected from the range of 100 μm to 1mm.

Electrochemical cells of the invention may further comprise one or moregraphene enclosures at least partially enclosing the active materialsfor positive and/or negative electrodes so as to provide overallenhanced mechanical, chemical and/or electronic properties of positiveand/or negative electrodes. Graphene layers of these embodiments mayfunction as chemical barriers and/or mechanical supports for thepositive and/or negative electrodes. In an embodiment, for example, theelectrochemical cell further comprises a first graphene layer at leastpartially enclosing the silicon active material of the negativeelectrode, a second graphene layer at least partially enclosing thesulfur active material of the positive electrode or both a firstgraphene layer at least partially enclosing the silicon active materialof the negative electrode and a second graphene layer at least partiallyenclosing the sulfur active material of the positive electrode. In anembodiment, for example, the graphene layer(s), such as the firstgraphene layer and the second graphene layer, are each permeable to thecharge carriers, such as Li⁺ ion charge carriers, for example, byproviding ion transport of charge carriers via defects or other passagestructures in the graphene layer(s). In an embodiment, for example, thegraphene layer(s), such as the first graphene layer and the secondgraphene layer, each independently have an average thickness selectedover the range of 5 nm to 100 nm. In an embodiment, the first graphenelayer is provided in physical contact with said silicon active materialor a layer provided thereon, and the second graphene layer is providedin physical contact with the sulfur active material or a layer providedthereon. In an embodiment, for example, the first graphene layer and/orthe second graphene layer provide an elastic barrier with theelectrolyte capable of accommodating expansion or contraction of thevolume of the silicon active material of the negative electrode and/orthe sulfur active material of the positive electrode, for example,occurring upon charge and discharge cycling. In an embodiment, forexample, the first graphene layer or the second graphene layerindependently provide a chemical barrier capable of prevent transport ofone or more reaction products from the silicon active materials of thenegative electrode or the sulfur active material of the positiveelectrode and the electrolyte. In an embodiment, for example, the secondgraphene layer prevents transport of polysulfides generated at thesulfur active material of the positive electrode to the electrolyte.

Electrochemical cells of the invention are compatible with a wide rangeof electrolytes, including liquid phase electrolytes, gel electrolytesor a solid phase electrolytes. In an embodiment, for example, theelectrolyte comprises a nonaqueous solvent and a lithium-containing saltat least partially dissolved in the nonaqueous solvent. Electrolytes ofthe invention may be provided between said electrodes such that chargecharges can be transported from the electrolyte to the electrodesurface, such as provided in physical contact with the electrode surfaceor a layer on the electrode surface (e.g., SEI layer). In an embodiment,for example, the electrolyte has an ionic conductivity for the chargecarriers, such as Li⁺ charge carriers, greater than or equal to 1.5 Scm⁻¹, and optionally greater than or equal to 5 S cm⁻¹. Useful lithiumsalts for this aspect of the present invention include, but are notlimited to, LiBF₄, LiF, LiClO₄, LiAsF₆, LiSbF₆ and LiPF₆. Solventsuseful in nonaqueous electrolytes of the present invention include, butare not limited to, propylene carbonate, 1,2-dimethoxy ethane,trifluoroethyl ether, diethyl ether, diethoxyethane, 1-3 dioxolane,tetrahydrofuran, 2-methyl tetrahydrofuran, ethylene carbonate, dimethylcarbonate, diethyl carbonate, ethyl methyl carbonate, methyl formate,α-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl ethylcarbonate, gamma-butyrolactone, diethoxyethane, acetonitrile, andmethylacetate. In an embodiment, for example, the electrolyte comprisesLiBF₄ at least partially dissolved in propylene carbonate or LiPF₆ atleast partially dissolved in ethylene carbonate.

The electrochemical systems of the present invention include secondaryelectrochemical cells and supercapacitors exhibiting good electronicperformance. In an embodiment, for example, an electrochemical system ofthe invention comprises a lithium ion battery, such as a silicon—sulfurlithium ion battery. In an embodiment, for example, the inventionprovides an electrochemical cell having a specific energy greater thanor equal to about 387.5 Wh kg⁻¹; e.g., ¼ of the theoretical specificenergy. In an embodiment, for example, the invention provides anelectrochemical cell having a standard cell voltage equal to or greaterthan 1.35 V; e.g., 80% voltage efficiency. In an embodiment, forexample, the invention provides an electrochemical cell having a cyclelife equal to or greater than about 1000 cycles.

In another aspect, the invention provides a method of generating anelectrical current, the method comprising the steps of: (i) providing anelectrochemical cell; the electrochemical comprising: (1) a negativeelectrode comprising a first assembly of carbon nanotubes supporting asilicon active material; (2) a positive electrode comprising a secondassembly of carbon nanotubes supporting a sulfur active material; and(3) an electrolyte provided between the positive electrode and thenegative electrode; the electrolyte capable of conducting, chargecarriers; wherein the first assembly of carbon nanotubes and the secondassembly of carbon nanotubes are physically separated from each otherand are supported by a common surface; and (ii) discharging theelectrochemical cell. In an embodiment, the method of this aspectfurther comprises any of the additional process steps and/or processconditions as described herein.

In another aspect, the invention provides a method of making anelectrochemical cell comprising the steps of: (i) providing a firstassembly of carbon nanotubes; (ii) incorporating a silicon activematerial into the first assembly of carbon nanotubes, thereby generatinga negative electrode comprising the silicon active material supported bythe first assembly of carbon nanotubes; (iii) providing a secondassembly of carbon nanotubes; (iv) incorporating a sulfur activematerial into the second assembly of carbon nanotubes, therebygenerating a positive electrode comprising the sulfur active materialsupported by the second assembly of carbon nanotubes; (v) providing asubstrate having an external surface; (vi) providing the negativeelectrode to a first region supported by the external surface; (vii)providing the positive electrode to a second region supported by theexternal surface; wherein the first assembly of carbon nanotubes of thenegative electrode and the second assembly of carbon nanotubes of thepositive electrode are physically separated from each other and aresupported by the external surface; and (viii) providing an electrolytebetween the positive electrode and the negative electrode; theelectrolyte capable of conducting charge carriers. In an embodiment, themethod of this aspect does not include a step of providing a separator(e.g., a permeable membrane) between the negative electrode and positiveelectrode. In an embodiment, the method of this aspect further comprisesany of the additional process steps and/or process conditions asdescribed herein.

In an embodiment, for example, the step of providing the first assemblyof carbon nanotubes comprises growing the first assembly of carbonnanotubes directly on the external surface of the substrate or astructure provided thereon. In an embodiment, for example, the step ofproviding the first assembly of carbon nanotubes comprises growing thefirst assembly of carbon nanotubes on a first current collector or on afirst electrical interconnect supported by the first current collectorand wherein the step of providing the second assembly of carbonnanotubes comprises growing the second assembly of carbon nanotubes on asecond current collector or on a second electrical interconnectsupported by the second current collector. Useful methods for growingthe first and second assembly of nanotubes include patterning a currentcollector or electrical interconnect with a carbon nanotube growthcatalyst and exposing the patterned nanotube growth catalyst to a gasphase or liquid phase carbon nanotube precursor. Patterning of carbonnanotube growth catalyst in these embodiments provides an effectivemeans of controlling the physical dimensions, physical properties (e.g.,surface concentration and/or density of nanotubes, etc.) and position ofthe carbon nanotube arrays. In an embodiment, for example, arrays ofvertically aligned carbon nanotubes are independently grown on thesubstrate, the first or second current collectors or electricalinterconnects, for example, via techniques known in the art [see, e.g.,Aria, A. I. & Gharib, M. Reversible Tuning of the Wettability of CarbonNanotube Arrays: The Effect of Ultraviolet/Ozone and Vacuum PyrolysisTreatments. Langmuir 27, 9005-9011, doi:10.1021/la201841m (2011)]. In anembodiment, for example, first and second carbon nanotube assemblies aregrown directly on the first and second current collectors, respectively,optionally in a configuration supported by the external surface of thesubstrate.

In an embodiment, for example, the step of incorporating the siliconactive material into the first assembly of carbon nanotubes andincorporating the sulfur active material into the second assembly ofcarbon nanotubes are independently carried out using a method selectedfrom the group consisting of physical vapor deposition, chemical vapordeposition, sputtering, electrodeposition, solution casting, liquidinfusion and liquid deposition. Incorporating silicon into the firstassembly of carbon nanotubes, for example, can be carried out viachemical vapor deposition using a silane precursor, dispersion ofsilicon nanoparticles and/or nanowires, and/or physical vapor depositionof Si using sputtering and/or evaporation techniques. Incorporatingsulfur into the second assembly of carbon nanotubes, for example, can becarried out via infusion of molten sulfur and/or sulfur containingsolution into the intertube spacing of the carbon nanotube assembly. Inembodiment, for example, incorporation of sulfur into the secondassembly of carbon nanotubes results in a change in mass selected overthe range of 300% to 350%.

Methods of this aspect of the invention may further compriseprelithiating the silicon active material supported by the firstassembly of carbon nanotubes and/or prelithiating the sulfur activematerial supported by the second assembly of carbon nanotubes. In someembodiments, for example, the silicon active material supported by thefirst nanotube array is placed in physical contact with metalliclithium, such as a lithium foil or stabilized lithium metal powder, toprovide for spontaneous prelithiation. Alternatively, prelithiation canbe carried out electrochemically, for example, by connecting the siliconactive material supported by the first nanotube array to the negativeside of a DC power supply while a lithium metal foil is connected to thepositive side. In an embodiment, the silicon active material supportedby the first nanotube array is prelithiated such that it undergoes achange in volume selected over the range of 250% to 300%.

In an embodiment, for example, the method of this aspect furthercomprises at least partially enclosing, and optionally fully enclosing,the silicon active material supported by the first assembly of carbonnanotubes with a first graphene layer and/or at least partiallyenclosing, and optionally fully enclosing, the sulfur active materialsupported by the second assembly of carbon nanotubes with a secondgraphene layer. Fabrication approaches for achieving graphene enclosuresuseful in the present methods include drop-casting and drying adispersion of graphene films directly onto the active material supportedby the nanotube assembly.

In an embodiment, for example, the method of this aspect furthercomprises providing the negative electrode and the positive electrode ona common surface comprising the external surface of the substrate or anintermediate structure provide thereon (e.g., a current collector,electrical interconnect, etc.). Device fabrication in some methods isachieved by providing the first and second current collectors supportingfirst and second carbon nanotube assemblies, respectively, onto anexternal surface of the substrate or an intermediate structure providedtherein (e.g., a current collector, electrical interconnect, etc.). Inan embodiment, for example, the first and second carbon nanotubeassemblies are provided on a common surface in a configuration withouttouching each other. Methods of this aspect of the invention may furthercomprise providing the positive and negative electrodes supported by thesubstrate in a casing.

Without wishing to be bound by any particular theory, there may bediscussion herein of beliefs or understandings of underlying principlesrelating to the devices and methods disclosed herein. It is recognizedthat regardless of the ultimate correctness of any mechanisticexplanation or hypothesis, an embodiment of the invention cannonetheless be operative and useful.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A: Schematic cross-sectional view of an electrochemical cell.

FIG. 1B: Schematic cross-sectional view of another electrochemical cell.

FIG. 1C: Schematic top view of an electrochemical cell in which theelectrodes form two adjacent strips.

FIG. 1D: Schematic top view of an electrochemical cell with interleavedelectrodes.

FIG. 1E: Schematic top view of another electrochemical cell withinterleaved electrodes; the negative electrode is generally c-shaped.

FIG. 1F: Schematic top view of another electrochemical cell in whicheach electrode is generally in the form of a rectangular spiral.

FIG. 2A. A schematic diagram showing top and side views of a separatorfree silicon—sulfur battery. A shown in this figure, the battery usescarbon nanotubes—graphene hybrid structures as the scaffolds andenclosures, and silicon and sulfur as the active electrode materials.The cathode and anode are patterned in a specific configuration suchthat both of them are on the same plane but without touching each other,eliminating the need to use separator.

FIG. 2B. A schematic diagram showing an electrode configuration usefulfor certain embodiments of the invention.

FIG. 3. Flow chart to fabricate a separator free silicon—sulfur batteryusing carbon nanotubes—graphene hybrid structures as the scaffolds andenclosures, and silicon and sulfur as the active electrode materials.

FIG. 4. A schematic diagram showing advantages of using carbonnanotubes—graphene hybrid structures against typical thin filmconfiguration. Carbon nanotubes—graphene hybrid structures allow the useof very thick films of silicon and sulfur active materials withoutescalating their internal electrical resistance. They also allow siliconand sulfur to expand and contract freely during the charging anddischarging cycles, so that stress and strain induced by volume changeof silicon do not act against sulfur and vice versa.

DETAILED DESCRIPTION OF THE INVENTION

In general, the terms and phrases used herein have their art-recognizedmeaning, which can be found by reference to standard texts, journalreferences and contexts known to those skilled in the art. The followingdefinitions are provided to clarify their specific use in the context ofthe invention.

“Standard electrode potential” (E°) refers to the electrode potentialwhen concentrations of solutes are 1 M, the gas pressures are 1 atm andthe temperature is 25 degrees Celsius. As used herein standard electrodepotentials are measured relative to a standard hydrogen electrode.

“Charge carrier” refers to an ion provided in an electrochemical cellthat migrates between positive and negative electrodes during dischargeand charging of the electrochemical cell. Charge carriers may be presentin an electrolyte and/or electrode components of the electrochemicalcell. Charge carriers useful in electrochemical cells of the presentinvention include lithium ion (Li+).

“Active material” refers to material in an electrode that takes part inthe electrochemical reactions which store or delivery electrical energy.In some embodiments, active materials for positive electrode and/ornegative electrode independently comprise a host material, such as a Li⁺host material. Active materials useful for positive electrodes of theinvention include sulfur, for example, elemental sulfur. Activematerials useful for negative electrodes of the invention includesilicon and alloys thereof.

“Host material” refers to a material capable of accommodating lithiumions. In this context, accommodating includes insertion of lithium ionsinto the host material, intercalation of lithium ions into the hostmaterial and/or reaction of lithium ions with the host material.

“Intercalation” refers to refers to the process wherein an ion insertsinto a host material to generate an intercalation compound via ahost/guest solid state redox reaction involving electrochemical chargetransfer processes coupled with insertion of mobile guest ions, such aslithium ions. Major structural features of the host material arepreserved after insertion of the guest ions via intercalation. In somehost materials, intercalation refers to a process wherein guest ions aretaken up with interlayer gaps (e.g., galleries) of a layered hostmaterial.

The term “electrochemical cell” refers to devices and/or devicecomponents that convert chemical energy into electrical energy orelectrical energy into chemical energy. Electrochemical cells have twoor more electrodes (e.g., positive and negative electrodes) and anelectrolyte, wherein electrode reactions occurring at the electrodesurfaces result in charge transfer processes. Electrochemical cellsinclude, but are not limited to, primary batteries, secondary batteriesand electrolysis systems. General cell and/or battery construction isknown in the art, see e.g., U.S. Pat. Nos. 6,489,055, 4,052,539,6,306,540, Seel and Dahn J. Electrochem. Soc. 147(3) 892-898 (2000).

“Electrode” refers to an electrical conductor where ions and electronsare exchanged with electrolyte and an outer circuit. “Positiveelectrode” and “cathode” are used synonymously in the presentdescription and refer to the electrode having the higher electrodepotential in an electrochemical cell (i.e. higher than the negativeelectrode). “Negative electrode” and “anode” are used synonymously inthe present description and refer to the electrode having the lowerelectrode potential in an electrochemical cell (i.e. lower than thepositive electrode). Cathodic reduction refers to a gain of electron(s)of a chemical species, and anodic oxidation refers to the loss ofelectron(s) of a chemical species.

The term “capacity” is a characteristic of an electrochemical cell thatrefers to the total amount of electrical charge an electrochemical cell,such as a battery, is able to hold. Capacity is typically expressed inunits of ampere-hours. The term “specific capacity” refers to thecapacity output of an electrochemical cell, such as a battery, per unitweight. Specific capacity is typically expressed in units ofampere-hours kg⁻¹.

The term “discharge rate” refers to the current at which anelectrochemical cell is discharged. Discharge rate can be expressed inunits of ampere. Alternatively, discharge rate can be normalized to therated capacity of the electrochemical cell, and expressed as C/(X t),wherein C is the capacity of the electrochemical cell, X is a variableand t is a specified unit of time, as used herein, equal to 1 hour.

“Current density” refers to the current flowing per unit electrode area.

“Electrode potential” refers to a voltage, usually measured against areference electrode, due to the presence within or in contact with theelectrode of chemical species at different oxidation (valence) states.

“Electrolyte” refers to an ionic conductor which can be in the solidstate, the liquid state (most common) or more rarely a gas (e.g.,plasma). Electrochemical cells of some embodiments comprise alithium-containing salt at least partially dissolved in a non-aqueoussolvent.

“Carbon nanotube” and “nanotube” are used synonymously and refer toallotropes of carbon comprising one or more cylindrically configuredgraphene sheets. Carbon nanotubes include single walled carbon nanotubes(SWNTs) and multiwalled carbon nanotubes (MWNTs). Carbon nanotubestypically have small diameters (≈1-100 nanometers) and large lengths (upto several millimeters (e.g., 5 mm)), and therefore may exhibit lengthto diameter ratios≈10² to about 10⁸. The longitudinal dimension of ananotube is its length and the cross sectional dimension of a nanotubeis its diameter (or radius). Carbon nanotubes include semiconductingcarbon nanotubes, metallic carbon nanotubes, semi-metallic carbonnanotubes and mixtures of these.

“Vertically aligned nanotubes” refer to nanotubes that are provided inan orientation wherein their lengths extend away from a common surfaceoptionally in substantially the same direction. In some embodiments,vertically aligned nanotubes are provided in an orientation whereintheir lengths extend away from a common surface that is one or moreexternal surface(s) of a substrate or structure provide on a substrate,such as a current collector, electrical interconnect, other devicecomponent, etc. In some embodiments, vertically aligned nanotubes areprovided in an array geometry wherein adjacent nanotubes in the arrayare substantially aligned with each other. In some embodiments,vertically aligned nanotubes have lengths extending in verticaldirections (i.e., away from the common surface) that are parallel. Insome embodiments, for example, vertically aligned nanotubes have alinear geometry wherein their lengths assume a substantially straightconfiguration (i.e. with deviations from linearity equal to or less thanabout 20%). As used in this context, the term “parallel” refers to ageometry in which the lengths of carbon nanotubes are substantiallyequidistant from each other for at least a portion of the points alongtheir respective lengths and have the same direction or curvature. Theterm parallel is intended to encompass some deviation from absoluteparallelism. In one embodiment, for example longitudinally alignednanotubes have parallel spatial orientations relative to each other withdeviations from absolute parallelism that are less than 20 degrees,preferably for some applications deviations from absolute parallelismthat are less than 10 degrees, and more preferably for some applicationsdeviations from absolute parallelism that are less than 1 degrees.“Substantially aligned nanotubes” have lengths extending in verticaldirections that are aligned with respect to each other but not providedin an absolutely parallel configuration. In some embodiments, forexample, substantially aligned nanotubes have a partially lineargeometry wherein their lengths assume a configuration with deviationsfrom absolute linearity greater than about 10%, and in some embodimentswith deviations from absolute linearity greater than about 20%.

“Assembly of carbon nanotubes” refers to group of carbon nanotubes thatare spatially localized, for example, spatially localized on a region ofone or more external surface(s) of a substrate or a structure provide ona substrate, such as a current collector, electrical interconnect, otherdevice component, etc. Assemblies of carbon nanotubes include nanotubearrays, including arrays of substantially aligned nanotubes, nanotubesexhibiting a high degree of linearity, and vertically aligned nanotubes.Assemblies of carbon nanotubes include nanotube networks, includingnetworks comprising nanotubes provided in random or substantiallyaligned orientations, and networks characterized by a plurality ofnanotube crossings.

The expression “degree of linearity” refers to a characteristic of acarbon nanotube that reflects deviations in the center positions of thetube along its length as compared to a perfectly straight line that bestapproximates the shape of the nanotube. Carbon nanotubes exhibiting ahigh degree of linearity have a conformation that approximates aperfectly straight line. The expression high degree of linearity isintended to include, however, nanotube conformations having somedeviations from a perfectly straight line that best approximates theshape of the nanotube. In some embodiments, nanotubes exhibiting a highdegree of linearity have deviations from perfect linearity along theirentire lengths that are less than or equal to about 50 nanometers, andin embodiments useful for some applications have deviations from perfectlinearity along their entire lengths that are less than or equal toabout 10 nanometers. In some embodiments, nanotubes exhibiting a highdegree of linearity have deviations from perfect linearity that are lessthan or equal to about 50 nanometers per micron of length, and inembodiments useful for some applications have deviations from perfectlinearity that are less than or equal to about 5 nanometers per micronof length. The present invention provides nanotube arrays and relatedmethods of making nanotube arrays wherein at least 95% of the nanotubesin the array exhibit a high degree of linearity.

“Array of nanotubes” refers to a plurality of nanotubes having a spatialconfiguration wherein individual nanotubes in the array have selectedrelative positions and relative spatially orientations.

“Nanotube precursors” refers to materials that are used to generatecarbon nanotubes, for example by chemical vapor deposition processes,electrochemical synthesis process and pyrolytic processes. In someembodiments, nanotube precursors interact with carbon nanotube growthcatalyst to generate carbon nanotubes. Exemplary nanotube precursorsinclude hydrocarbons such as methane, carbon monoxide, ethylene,benzene, and ethyl alcohol.

“Nanotube growth catalysts” are materials that catalyze the formationand growth of carbon nanotubes. Useful nanotube growth catalysts for themethods of the present invention include, but are not limited to,ferritin, nickel, molybdenum, palladium, yttrium, iron, copper,molybdenum, cobalt.

“Supported by a substrate” refers to a structure that is present atleast partially on a substrate surface or present at least partially onone or more intermediate structures positioned between the structure andthe substrate surface. The term “supported by a substrate” may alsorefer to structures partially or fully embedded in a substrate,structures partially or fully immobilized on a substrate surface andstructures partially or fully laminated on a substrate surface.

The term “nanostructured” refers materials and/or structures have aplurality of discrete structural domains with at least one physicaldimension (e.g., height, width, length, cross sectional dimension) thatis less than about 1 micron. In this context, structural domains referto features, components or portions of a material or structure having acharacteristic composition, morphology and/or phase. “Supported by asubstrate” refers to a structure that is present at least partially on asubstrate surface or present at least partially on one or moreintermediate structures positioned between the structure and thesubstrate surface.

FIGS. 1A and 1B schematically illustrate electrochemical cells of thepresent invention. The cells illustrated in FIGS. 1A and 1B are shown incross-section. Each of the electrochemical cells in FIGS. 1A and 1Bcomprises a negative electrode (40) and a positive electrode (50)provided on a common surface (65) and supported by substrate (60).Negative electrode (40) and a positive electrode (50) are separated byelectrolyte (90) capable of conducting charge carriers, such as lithiumion (Li⁺) charge carriers. Negative electrode (40) and a positiveelectrode (50) independently comprise an assembly of nanotubessupporting an active material. In some embodiments, for example,negative electrode (40) comprises a first assembly of nanotubes (10)supporting a silicon active material (20) and positive electrode (50)comprises a second assembly of nanotubes (10) supporting a sulfur activematerial (30). FIGS. 1A and 1B also show optional components of negativeelectrode (40) and a positive electrode (50), including currentcollector (80) supported by substrate (60) and graphene enclosure (70)at least partially enclosing, and optionally in physical contact with,the active material supported by the carbon nanotube assemblies.Optionally, a graphene interconnection (not shown in FIGS. 1A and 1B butshown in FIG. 2B) is independently provided between the assembly ofnanotubes (10) and current collector (80) of either, or both of,positive or negative electrodes.

As shown in FIGS. 1A and 1B, the negative and positive electrodes are(40) and (50) separated by distance d₁ defining a space occupied byelectrolyte (90). In some embodiments, d₁ is greater than or equal to 10μm, and optionally for some embodiments d₁ is greater than or equal to100 μm. This arrangement allows ions to be effectively transportedbetween positive electrode (50) and negative electrode (40) duringcharge and discharge of the electrochemical cell but ensures thatpositive electrode (50) and negative electrode (40) are not inelectrical contact. Both electrodes (40) and (50) are maintained in aspatially localized position supported by common substrate (60), forexample, through their connection (e.g., bonding) to the substrate (60)or a structure supported by the substrate such as current collector (80)or an electrical interconnect provided on current collector (80). Theelectrode arrangement in FIGS. 1A and 1B, therefore, does not include,or require, a conventional separator (e.g. ion conducting permeablemembrane) to separate the positive and negative electrodes in the celldesigns illustrated in FIGS. 1A and 1B. Rather, separation of thenegative (40) and positive electrodes and (50) is effectively achievedvia spatially localizing their respective carbon nanotube assemblies,for example, via patterning of the nanotubes on the substrate or astructure supported by the substrate.

As shown in FIGS. 1A and 1B, each electrode comprises an assembly ofcarbon nanotubes (10). As used in description of this embodiment, anassembly of nanotubes refers to a plurality of nanotubes provided in aspatially localized region supported by substrate (60). As shown FIGS.1A and 1B, the individual nanotubes (10) are separated by an, averagedistance d2 in the negative electrode and a distance d3 in the positiveelectrode. In an embodiment, the average spacing between adjacentnanotubes is 2-4 times the average diameter of the nanotubes. In anembodiment, the average spacing between adjacent nanotubes is selectedover the range of 10 nm to 200 nm.

FIGS. 1A and 1B show the carbon nanotubes as being generally verticallyaligned. In the embodiment shown in FIGS. 1A and 1B, vertically alignedcarbon nanotubes are provided in an orientation that is substantiallyperpendicular to the surface on which they are supported or attached.Nanotubes are said to be substantially perpendicular when they areoriented on average within 30, 25, 20, 15, 10, or 5 degrees of thesurface normal. In other embodiments the carbon nanotubes are notvertically aligned. In an embodiment, non-aligned carbon nanotubes maytake the form of a network, mat or forest of carbon nanotubes grown fromthe catalyst material. In an embodiment, the nanotubes are multiwallednanotubes. In an embodiment, the average diameter of the multiwallednanotubes is from 15 nm to 400 nm, from 15 nm to 200 nm, from 15 nm to100 nm or from 15 nm to 50 nm, In an embodiment, the nanotubes are grownfrom a catalyst material, for example provided on a current collector(80) or an electrical interconnect supported by the substrate (60). Insome embodiments, the current collector (80) is a metal currentcollector, carbon current collector (e.g., graphite) or particulatecurrent collector. In an embodiment, the nanotubes are bonded to thecommon surface, to the catalyst material, or a combination thereof.Suitable catalyst materials include, but are not limited to Fe, Co, Ni,Au, Ag, Cu, Pb and In.

The assembly of nanotubes in each electrode supports an active material.In an embodiment the active material fills in the spaces between thenanotubes, as shown in FIG. 1A. In other embodiments the active materialdoes not completely fill the space between the carbon nanotubes, butrather at least partially coats individual nanotubes as shown in FIG.1B. A variety of techniques can be used for providing the activematerial onto the nanotubes of the assembly including physical vapordeposition, chemical vapor deposition, electrodeposition, sputtering,solution casting, liquid infusion and liquid deposition. In anembodiment, negative electrode (40) comprises a silicon active material(20) and positive electrode (50) comprises support for a sulfur activematerial (30). Use of a carbon nanotube array to support the activematerial increases the overall conductivity of the electrode, therebyproviding enhanced discharge rate capability. In an embodiment, use of acarbon nanotube array to support the active material reduces theresidual stress due to volume change of the active material duringcycling of the electrochemical cell, thereby enhancing cyclingperformance of the electrochemical cell.

In an aspect of the invention, a graphene layer (70) is connected to andat least partially encloses the active material of an electrode. Asshown in FIG. 1A, a graphene layer is connected to and at leastpartially encloses the sulfur active material, while another graphenelayer is connected to and at least partially encloses the silicon activematerial. In an embodiment, the graphene layer allows efficienttransport of lithium ions through the layer but limits, or preventstransport, of certain reaction products (e.g. polysulfides) fromelectrode to electrolyte, for example, reaction products that cannegatively impact electrochemical performance. The relative ease ofmovement of various species through the graphene layer can be controlledat least in part through control of the defect density in the graphenelayer. Limiting the extent of electrode degradation can also limitdegradation of the electrolyte and/or the interface (e.g. the SEI layer)between the electrolyte and the electrode.

An electrolyte (90) is present between the negative and positiveelectrodes. In an embodiment, the electrolyte is a liquid having highionic conductivity and low electrical conductivity. In an embodiment,the electrolyte is a nonaqueous electrolyte comprising a solution of alithium salt and a solvent. Useful lithium salts for this aspect of thepresent invention include, but are not limited to, LiBF₄, LiF, LiClO₄,LiAsF₆, LiSbF₆ and LiPF₆. In an embodiment, for example, the lithiumsalt, such as LiBF4, has a concentration in the nonaqueous electrolytesolution that is preferably less than 1.0 M for some applications, andmore preferably less than 0.5 M for some applications. Useful lithiumsalt concentrations for some electrochemical cells of the presentinvention are selected from the range of about 0.75 M to about 0.25 M,for example when LiBF4 is the selected lithium salt. Solvents useful innonaqueous electrolytes of the present invention include, but are notlimited to, propylene carbonate, 1,2-dimethoxy ethane, trifluoroethylether, diethyl ether, diethoxyethane, 1-3 dioxolane, tetrahydrofuran,2-methyl tetrahydrofuran, ethylene carbonate, dimethyl carbonate,diethyl carbonate, ethyl methyl carbonate, methyl formate,α-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl ethylcarbonate, gamma-butyrolactone, diethoxyethane, acetonitrile, andmethylacetate. Nonaqueous electrolytes of the present invention alsoinclude fluorine analogs of the solvents

An enclosure (100) is typically included in the cell but is not is notshown in FIGS. 1A and 1B to facilitate identification of various cellfeatures. Enclosure (100) is shown, however, in FIGS. 1C-1F and FIG. 2,as described below.

FIGS. 1C-F illustrate top views of different electrode configurations ofelectrochemical cells of the invention. In an embodiment, bothelectrodes are similarly shaped. FIG. 1C illustrates a configuration inwhich both electrodes are generally rectangular in shape. FIG. 1D theelectrodes are interleaved; one electrode comprises recessed featureswhile the other electrode comprises projecting features. FIG. 1Fillustrates a configuration in which both electrodes are generally inthe form of a rectangular spiral. FIG. 1E illustrates an embodiment inwhich one electrode is C-shaped while the other electrode is generallyrectangular; the two electrodes are interleaved.

Example 1: Carbon Nanotubes—Graphene Hybrid Structures for SeparatorFree Silicon-Sulfur Batteries

For decades, lithium ion batteries have been known as the most prominentmeans of storing electrical energy. However, a large-scale adoption ofthese batteries has been largely hindered by safety concerns, highproduction and maintenance cost, and mediocre performance. Such issuesare mostly originated from the limited energy density and poorcharge-discharge cycles associated with the currently available positiveand negative electrodes (cathodes and anodes respectively). For example,the commonly used LiMn₂O₄ cathodes have a very low energy density andlimited charge-discharge cycles. While LiCoO₂ and LiFePO₄ cathodes havea reasonably high energy density, the former are extremely toxic and thelater have a very poor electrical conductivity. Similarly, the use oflithium metal anodes often leads to fire and explosion hazards.Dendrites grown on lithium metal during the charge-discharge cycles maycause a short circuit and thermal runaway. While the commonly usedgraphite anodes are very cheap and highly conductive, their energydensity is extremely poor.

In order to mitigate the safety concern and improve the energy densityof lithium ion batteries, the use of prelithiated silicon and sulfur asanode and cathode respectively has been envisioned.^(1,2) Such acombination eliminates the need of using lithium metal andinsertion-compound electrodes that are highly unstable and have a poorenergy density. Because of its abundance and high theoretical capacity(4200 mAhg⁻¹), silicon has been considered the most promising anodematerial. However, the use of prelithiated silicon anode is not freefrom problems. Silicon undergoes a huge volume change during thecharge-discharge cycle. Fractures, cracks, and disconnection fromcurrent collector due to such volume change lead to a rapid capacityfading and poor cycle life. Sulfur has also been considered the mostpromising cathode material because of its abundance, low cost, and hightheoretical capacity (1675 mAhg⁻¹). In addition, sulfur is also moreenvironmentally friendly compared to the toxic transition-metalcompounds. Nevertheless, the use of a sulfur cathode is also not freefrom problems. Sulfur undergoes a series of structural and morphologicalchanges during the charge-discharge cycle involving the formation ofsoluble lithium polysulfides Li₂S_(x) and insoluble sulfides Li₂S₂/Li₂Sin liquid electrolyte. Such structural changes along with the highresistances of sulfur result in unstable electrochemical contact withinsulfur electrodes. These issues lead to a rapid capacity fading, poorcycle life, low system efficiency, and large internal resistance.

It has been predicted that a lithium ion battery using a combination ofsilicon anode and sulfur cathode will have a specific energy of 1550Wh/kg.³ However, the actual battery performance may not be as impressiveonce the weight of other battery components, e.g. current collectors,electrolytes, separators, connectors, casing and packaging, has beentaken into account. Typically, the actual specific energy of a batteryis about a factor of 4 to 12 time smaller than the specific energy ofjust the active electrode materials.⁴ Thus, in order to achieve anenhanced actual battery performance, the mass ratio (MR) and volumeratio (VR) between the active electrode materials and the other batterycomponents has to be maximized.

In this Example, a novel scheme is described to fabricate highperformance yet affordable lithium ion batteries using carbonnanotubes—graphene hybrid structures as the scaffolds and enclosures,and silicon and sulfur as the active electrode materials. Thesebatteries will also be inherently safe due to the absence of toxic andunstable materials. This scheme includes four fundamental aspects:

1. Use of prelithiated silicon and sulfur as the active anode andcathode materials respectively.

2. Use of vertically aligned carbon nanotubes (VACNT) as the electrodescaffolds to accommodate a large volume change of silicon and sulfurduring charge-discharge cycles. These VACNT scaffolds allow the use ofvery thick layers of silicon and sulfur while simultaneously reducingtheir internal electrical resistance.

3. Use of graphene as the electrode enclosures to prevent dissolution ofpolysulfides into the electrolytes as well as to minimize fracture ofsilicon and sulfur due to a volume change.

4. Cathode and anode, as well as their corresponding current collectors,are patterned so that both of them are placed on the same plane and theneed for a separator can be eliminated.

In an embodiment, a patterned carbon nanotube scaffold comprising anassembly of carbon nanotubes is independently provided to support activematerials for positive and negative electrodes, and an optionally agraphene enclosure partially encloses the active material. In anembodiment, patterning of the assemblies of carbon nanotubes spatiallylocalizes positive and negative electrodes to separate regions supportedby a substrate, thereby eliminating the need for a conventionalseparator component. Prelithiated active materials are used in someembodiments, thereby eliminating the need for metallic lithium to avoidproblems associated with dendrite formation during cycling.

As described earlier, in some embodiments both cathode and anode arepatterned in a specific configuration such that both of them are on thesame plane without touching each other. One example of thisconfiguration is shown in FIG. 2A providing a schematic diagram showingtop and side views of a separator free silicon—sulfur battery. A shownin this figure, the battery uses carbon nanotubes—graphene hybridstructures as the scaffolds and enclosures, and silicon and sulfur asthe active electrode materials. The cathode and anode are patterned in aspecific configuration such that both of them are on the same plane butwithout touching each other, eliminating the need to use separator. FIG.2B provides an alternative electrode configuration include a grapheneinterconnect provided between the carbon nanotube assembly and thecurrent collector, for example, to minimize ESR and protect the currentcollector (e.g., Cu foil).

FIG. 3 provides a flow chart describing a process to fabricate aseparator free silicon—sulfur battery using carbon nanotubes—graphenehybrid structures as the scaffolds and enclosures, and silicon andsulfur as the active electrode materials. Since the shape and positionof both cathode and anode is dictated by that of VACNT scaffolds, it isbeneficial for some applications to start the fabrication process bypatterning the current collector and grow the VACNT scaffolds directlyon it. The metal current collector can be patterned using well knownmicrofabrication techniques such as lithography, physical vapordeposition, and electroplating. VACNT scaffolds can then be directlygrown on the current collector by chemical vapor deposition.⁵ In anembodiment, metal current collectors are patterned on a substrate andVACNT assemblies are grown on the current collectors, for example, byexposing a patterned nanotube catalyst to a nanotube precursor. Next,active materials for positive and negative electrodes are provided tothe nanotube assemblies, and optionally electrochemically prelithiated.Optionally, separate graphene enclosures are provided at least partiallyenclosing the active materials of the positive and negative electrodes.Alternatively, for some embodiments, VACNT assemblies are generated ongraphene films which are subsequently transferred to be in electricalcontact with current collectors patterned onto a substrate to fabricatethe electrochemical cell.

FIG. 4 provides a schematic diagram showing advantages of using carbonnanotubes—graphene hybrid structures as compared to a conventional thinfilm configuration. Carbon nanotubes—graphene hybrid structures allowthe use of very thick films of silicon and sulfur active materialswithout escalating their internal electrical resistance. They also allowsilicon and sulfur to expand and contract freely during the charging anddischarging cycles, so that stress and strain induced by volume changeof silicon do not act against sulfur and vice versa. Advantages of thepresent electrode configurations include: (i) the ability to allow Siand S active materials to expand and contract freely, (ii) forcesinduced by volume change of Si and S active materials do not act againsteach other, thereby preventing a short circuit to occur, (ii) betterelectrical conductivity and thermal dissipation, thereby allowing highloading of Si and S active materials to achieve higher capacity andminimizing risk of thermal runaway.

It is important to note that VACNT scaffolds allow the use of very thickfilms of silicon and sulfur active materials, up to several mm (e.g., 3mm), without escalating their internal electrical resistance (FIG. 4).The aforementioned configuration also allows the need for a separator,typically placed between anode and cathode, to be completely eliminated.The high loading of silicon and sulfur along with the absence of aseparator result in a high value of MR and VR, which ultimately bringthe actual specific energy of the battery closer to the theoreticalspecific energy of just the active electrode materials. In addition,this configuration allows silicon and sulfur to expand and contractfreely during the charging and discharging cycles. Thus, stress andstrain induced by volume change of the cathode do not act against theanode and vice versa (FIG. 4).

Silicon can be incorporated into VACNT scaffolds using previouslypublished methods (FIG. 3). These methods include chemical vapordeposition using silane precursor,^(6,7) dispersion of siliconnanoparticles and nanowires,⁸ and physical vapor deposition usingsputtering and evaporation.⁹ Once the silicon has been successfullyincorporated into VACNT (Si-VACNT), the next step is the insertion oflithium ion into the silicon by electrochemical reaction (FIG. 3). Herethe Si-VACNT is connected to a lithium metal foil or stabilized lithiummetal powders (SLMP) in the presence of electrolyte.^(1,10) Ideally, theprelithiation process occurs spontaneously when the Si-VACNT is indirect contact with lithium metal foil or SLMP. However, overpotentialcan also be used when the Si-VACNT is not readily intercalated bylithium ions upon contact. In this case, the Si-VACNT is charged byconnecting it to the negative side of a DC power supply while a lithiummetal foil is connected to the positive side. Care must be taken so thatthe Si-VACNT is not overlithiated, which may result in fracture and lossof structural integrity. In principle, a successful prelithiationprocess of Si-VACNT is indicated by a volume change of about 250%-300%.

Once Si-VACNT electrodes have been successfully prelithiated, they canthen be encapsulated by graphene enclosures to improve the cycle lifeand the overall battery efficiency (FIG. 3). Graphene enclosures can bedeposited onto Si-VACNT by simply drop-casting and drying a dispersionof graphene films directly onto the anodes. A dispersion of graphenefilms in a liquid can be made using previously publishedmethods.^(11,12) During the charge-discharge cycles the grapheneenclosures constrain the volume change of the Si-VACNT, preventingsilicon from being fractured and disconnected from the VACNT scaffolds.Since graphene enclosures are electrically and ionically conductive,they minimize the internal resistance of the Si-VACNT whilesimultaneously allowing electrochemical reaction to occur. The grapheneencapsulated Si-VACNT (Si-VACNT/G) electrodes are readily used as anodesin lithium ion batteries.

Similarly, sulfur can be incorporated into VACNT scaffolds usingpreviously published methods (FIG. 3). These methods include infusion ofmolten sulfur and/or sulfur containing solutions into the inter-tubespacing of VACNT.¹³⁻¹⁵ In principle, a successful sulfur infusionprocess is indicated by a weight change of about 300%-350%. Once VACNTscaffolds have been completely infused by sulfur (S-VACNT), they canthen be encapsulated by graphene enclosures. During the charge-dischargecycles the graphene enclosures prevent a direct contact betweenelectrolyte and polysulfides formed in the S-VACNT. Thus, dissolution ofpolysulfides into the electrolyte can be avoided while simultaneouslyallowing electrochemical reaction to occur. This ultimately improvescycle life and minimizes the rapid fading in capacity of the S-VACNT. Inaddition, graphene enclosures also reduce the internal resistance of theS-VACNT to ultimately improve the overall battery efficiency. Thegraphene encapsulated S-VACNT (S-VACNT/G) electrodes are readily used asanodes in lithium ion batteries.

REFERENCES

-   1 Liu, N., Hu, L., McDowell, M. T., Jackson, A. & Cui, Y.    Prelithiated Silicon Nanowires as an Anode for Lithium Ion    Batteries. ACS Nano 5, 6487-6493, doi:10.1021/nn2017167 (2011).-   2 He, G., Ji, X. & Nazar, L. High “C” rate Li—S cathodes: sulfur    imbibed bimodal porous carbons. Energy & Environmental Science 4,    2878-2883, doi:10.1039/C1EE01219C (2011).-   3 Yang, Y. et al. New Nanostructured Li2S/Silicon Rechargeable    Battery with High Specific Energy. Nano letters 10, 1486-1491,    doi:10.1021/nl100504q (2010).-   4 Gogotsi, Y. & Simon, P. True Performance Metrics in    Electrochemical Energy Storage. Science 334, 917-918,    doi:10.1126/science.1213003 (2011).-   5 Aria, A. I. & Gharib, M. Reversible Tuning of the Wettability of    Carbon Nanotube Arrays: The Effect of Ultraviolet/Ozone and Vacuum    Pyrolysis Treatments. Langmuir 27, 9005-9011, doi:10.1021/la201841m    (2011).-   6 Evanoff, K. et al. Towards Ultrathick Battery Electrodes: Aligned    Carbon Nanotube—Enabled Architecture. Advanced Materials 24,    533-537, doi:10.1002/adma.201103044 (2012).-   7 Forney, M. W. et al. High performance silicon free-standing anodes    fabricated by low-pressure and plasma-enhanced chemical vapor    deposition onto carbon nanotube electrodes. Journal of Power Sources    228, 270-280, doi:http://dx.doi.org/10.1016/j.jpowsour.2012.11.109    (2013).-   8 Cui, L.-F., Hu, L., Choi, J. W. & Cui, Y. Light-Weight    Free-Standing Carbon Nanotube-Silicon Films for Anodes of Lithium    Ion Batteries. ACS Nano 4, 3671-3678, doi:10.1021/nn100619m (2010).-   9 Katar, S. et al. Silicon Encapsulated Carbon Nanotubes. Nanoscale    Res. Lett. 5, 74-80 (2009).-   10 Forney, M. W., Ganter, M. J., Staub, J. W., Ridgley, R. D. &    Landi, B. J. Prelithiation of Silicon-Carbon Nanotube Anodes for    Lithium Ion Batteries by Stabilized Lithium Metal Powder (SLMP).    Nano Letters 13, 4158-4163, doi:10.1021/nl401776d (2013).-   11 Zhang, X. et al. Dispersion of graphene in ethanol using a simple    solvent exchange method. Chemical Communications 46, 7539-7541,    doi:10.1039/C00002688C (2010).-   12 Pu, N.-W. et al. Dispersion of graphene in aqueous solutions with    different types of surfactants and the production of graphene films    by spray or drop coating. Journal of the Taiwan Institute of    Chemical Engineers 43, 140-146,    doi:http://dx.doi.org/10.1016/j.jtice.2011.06.012 (2012).-   13 Zheng, G., Yang, Y., Cha, J. J., Hong, S. S. & Cui, Y. Hollow    Carbon Nanofiber-Encapsulated Sulfur Cathodes for High Specific    Capacity Rechargeable Lithium Batteries. Nano Letters 11, 4462-4467,    doi:10.1021/n12027684 (2011).-   14 Su, Y.-S., Fu, Y. & Manthiram, A. Self-weaving sulfur-carbon    composite cathodes for high rate lithium-sulfur batteries. Physical    Chemistry Chemical Physics 14, 14495-14499, doi:10.1039/C2CP42796F    (2012).-   15 Dorfler, S. et al. High capacity vertical aligned carbon    nanotube/sulfur composite cathodes for lithium-sulfur batteries.    Chemical Communications 48, 4097-4099, doi:10.1039/C2CC17925C    (2012).

STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS

All references cited throughout this application, for example patentdocuments including issued or granted patents or equivalents; patentapplication publications; and non-patent literature documents or othersource material; are hereby incorporated by reference herein in theirentireties, as though individually incorporated by reference, to theextent each reference is at least partially not inconsistent with thedisclosure in this application (for example, a reference that ispartially inconsistent is incorporated by reference except for thepartially inconsistent portion of the reference).

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding any equivalents ofthe features shown and described or portions thereof, but it isrecognized that various modifications are possible within the scope ofthe invention claimed. Thus, it should be understood that although thepresent invention has been specifically disclosed by preferredembodiments, exemplary embodiments and optional features, modificationand variation of the concepts herein disclosed may be resorted to bythose skilled in the art, and that such modifications and variations areconsidered to be within the scope of this invention as defined by theappended claims. The specific embodiments provided herein are examplesof useful embodiments of the present invention and it will be apparentto one skilled in the art that the present invention may be carried outusing a large number of variations of the devices, device components,and method steps set forth in the present description. As will beobvious to one of skill in the art, methods and devices useful for thepresent methods can include a large number of optional composition andprocessing elements and steps.

When a group of substituents is disclosed herein, it is understood thatall individual members of that group and all subgroups, including anyisomers, enantiomers, and diastereomers of the group members, aredisclosed separately. When a Markush group or other grouping is usedherein, all individual members of the group and all combinations andsubcombinations possible of the group are intended to be individuallyincluded in the disclosure. When a compound is described herein suchthat a particular isomer, enantiomer or diastereomer of the compound isnot specified, for example, in a formula or in a chemical name, thatdescription is intended to include each isomers and enantiomer of thecompound described individually or in any combination. Additionally,unless otherwise specified, all isotopic variants of compounds disclosedherein are intended to be encompassed by the disclosure. For example, itwill be understood that any one or more hydrogens in a moleculedisclosed can be replaced with deuterium or tritium. Isotopic variantsof a molecule are generally useful as standards in assays for themolecule and in chemical and biological research related to the moleculeor its use. Methods for making such isotopic variants are known in theart. Specific names of compounds are intended to be exemplary, as it isknown that one of ordinary skill in the art can name the same compoundsdifferently.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural reference unless thecontext clearly dictates otherwise. Thus, for example, reference to “acell” includes a plurality of such cells and equivalents thereof knownto those skilled in the art, and so forth. As well, the terms “a” (or“an”), “one or more” and “at least one” can be used interchangeablyherein. It is also to be noted that the terms “comprising”, “including”,and “having” can be used interchangeably. The expression “of any ofclaims XX-YY” (wherein XX and YY refer to claim numbers) is intended toprovide a multiple dependent claim in the alternative form, and in someembodiments is interchangeable with the expression “as in any one ofclaims XX-YY.”

Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of ordinary skillin the art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are now described. Nothing herein is to be construed as anadmission that the invention is not entitled to antedate such disclosureby virtue of prior invention.

Whenever a range is given in the specification, for example, a range ofintegers, a temperature range, a time range, a composition range, orconcentration range, all intermediate ranges and subranges, as well asall individual values included in the ranges given are intended to beincluded in the disclosure. As used herein, ranges specifically includethe values provided as endpoint values of the range. As used herein,ranges specifically include all the integer values of the range. Forexample, a range of 1 to 100 specifically includes the end point valuesof 1 and 100. It will be understood that, any subranges or individualvalues in a range or subrange that are included in the descriptionherein can be excluded from the claims herein.

As used herein, “comprising” is synonymous and can be usedinterchangeably with “including,” “containing,” or “characterized by,”and is inclusive or open-ended and does not exclude additional,unrecited elements or method steps. As used herein, “consisting of”excludes any element, step, or ingredient not specified in the claimelement. As used herein, “consisting essentially of” does not excludematerials or steps that do not materially affect the basic and novelcharacteristics of the claim. In each instance herein any of the terms“comprising”, “consisting essentially of” and “consisting of” can bereplaced with either of the other two terms. The inventionillustratively described herein suitably can be practiced in the absenceof any element or elements, limitation or limitations which is notspecifically disclosed herein.

One of ordinary skill in the art will appreciate that startingmaterials, biological materials, reagents, synthetic methods,purification methods, analytical methods, assay methods, and biologicalmethods other than those specifically exemplified can be employed in thepractice of the invention without resort to undue experimentation. Allart-known functional equivalents, of any such materials and methods areintended to be included in this invention. The terms and expressionswhich have been employed are used as terms of description and not oflimitation, and there is no intention in the use of such terms andexpressions of excluding any equivalents of the features shown anddescribed or portions thereof, but it is recognized that variousmodifications are possible within the scope of the invention claimed.Thus, it should be understood that although the invention has beenspecifically disclosed by preferred embodiments and optional features,modification and variation of the concepts herein disclosed can beresorted to by those skilled in the art, and that such modifications andvariations are considered to be within the scope of this invention asdefined by the appended claims.

We claim:
 1. An electrochemical cell comprising: a negative electrodecomprising a first assembly of carbon nanotubes supporting a siliconactive material filling spaces between the nanotubes; a positiveelectrode comprising a second assembly of carbon nanotubes supporting asulfur active material; an electrolyte provided between said positiveelectrode and said negative electrode; said electrolyte capable ofconducting charge carriers; and a substrate; wherein said first assemblyof carbon nanotubes and said second assembly of carbon nanotubes arephysically separated from each other and are supported by saidsubstrate; and wherein said first assembly of carbon nanotubes isprovided as one or more first strips supported by said substrate andsaid second assembly of carbon nanotubes is provided as one or moresecond strips supported by said substrate, said one or more first stripsand said one or more second strips arranged in a space filling geometryselected from a group of geometries consisting of an interleavedgeometry, a nested geometry, a coiled geometry, and a spiral geometry.2. The electrochemical cell of claim 1, wherein said charge carriers areLi+ ions; and wherein said positive electrode and said negativeelectrode accommodate said Li+ ions during charge or discharge of saidelectrochemical cell.
 3. The electrochemical cell of claim 1, whereinsaid silicon active material, said sulfur active material or both areprelithiated.
 4. The electrochemical cell of claim 1, wherein saidelectrochemical cell does not include a separator.
 5. Theelectrochemical cell of claim 1, wherein said first assembly of carbonnanotubes and said second assembly of carbon nanotubes are physicallyseparated from each other by at least 10 μm.
 6. The electrochemical cellof claim 1, wherein said first assembly of carbon nanotubes is providedon a first current collector supported by said external surface of saidsubstrate and said second assembly of carbon nanotubes is provided on asecond current collector supported by said external surface of saidsubstrate.
 7. The electrochemical cell of claim 6, further comprising afirst graphene electrical interconnect and a second grapheneinterconnect, wherein said first graphene electrical interconnect isprovided between said first assembly of carbon nanotubes and said firstcurrent collector; and wherein said second graphene electricalinterconnect is provided between said second assembly of carbonnanotubes and said second current collector.
 8. The electrochemical cellof claim 1, wherein said first strips are separated from said secondstrips by at least 10 μm; and wherein said first strips and said secondstrips are characterized by widths selected from the range of 10 μm to 1mm and lengths selected from the range of 30 μm to 3 mm.
 9. Theelectrochemical cell of claim 1, wherein said carbon nanotubes of saidfirst assembly and said second assembly comprise single walled carbonnanotubes, multiwalled carbon nanotubes, metallic carbon nanotubes orany combination of these; and wherein said carbon nanotubes of saidfirst assembly and said second assembly are independently characterizedby radial dimensions selected over the range of 5 nm to 100 nm, lengthdimensions selected over the range of 10 μm to 5 mm and an averagesurface concentration greater than or equal to 25 nanotubes per μm⁻².10. The electrochemical cell of claim 1, wherein said carbon nanotubesof said first assembly and said second assembly comprise one or morecarbon nanotube arrays or carbon nanotube networks.
 11. Theelectrochemical cell of claim 1, wherein said carbon nanotubes of saidfirst assembly comprise a first array of vertically aligned carbonnanotubes and said carbon nanotubes of said second assembly comprise asecond array of vertically aligned carbon nanotubes.
 12. Theelectrochemical cell of claim 11, wherein said vertically aligned carbonnanotubes of said first array and said second array extend in one ormore directions away from said common surface.
 13. The electrochemicalcell of claim 11, wherein said vertically aligned carbon nanotubes ofsaid first array and said second array extend in a common direction awayfrom said common surface.
 14. The electrochemical cell of claim 11,wherein said vertically aligned carbon nanotubes of said first array andsaid second array are independently characterized by an averageinterspacing between adjacent nanotubes selected over the range of 10 nmto 200 nm.
 15. The electrochemical cell of claim 1, wherein said carbonnanotubes of said first assembly provide a mechanical scaffold capableof accommodating stress resulting from expansion of said silicon activematerial or said sulfur active material during charging or discharge ofsaid electrochemical cell so as to allow a reversible change in volumeof said negative electrode or said positive electrode greater than orequal to 200% without mechanical failure.
 16. The electrochemical cellof claim 1, wherein said silicon active material comprises elementalsilicon or an alloy thereof and wherein said sulfur active materialcomprises elemental sulfur.
 17. The electrochemical cell of claim 1,wherein said silicon active material and said sulfur active materialindependently comprise a single crystalline material, a polycrystallinematerial or amorphous material and wherein said silicon active materialis provided on said carbon nanotubes of said first assembly or saidsulfur active material is provided on said carbon nanotubes of saidsecond assembly by a process selected from the group consisting physicalvapor deposition, chemical vapor deposition, sputtering,electrodeposition, solution casting, liquid infusion and liquiddeposition.
 18. The electrochemical cell of claim 1, wherein saidsilicon active material at least partially coats said carbon nanotubesof said first assembly, said sulfur active material at least partiallycoats said carbon nanotubes of said second assembly or wherein saidsilicon active material at least partially coats said carbon nanotubesof said first assembly and said sulfur active material at leastpartially coats said carbon nanotubes of said second assembly.
 19. Theelectrochemical cell of claim 1, wherein said silicon active materialprovides a coating on at least a portion of said carbon nanotubes ofsaid first assembly having a thickness greater than or equal to 0.1 μmor wherein said sulfur active material provides a coating on at least aportion of said carbon nanotubes of said second assembly having athickness greater than or equal to 0.1 μm.
 20. The electrochemical cellof claim 1, further comprising a first graphene layer at least partiallyenclosing said silicon active material of said negative electrode, asecond graphene layer at least partially enclosing said sulfur activematerial of said positive electrode or both.
 21. The electrochemicalcell of claim 20, wherein said first graphene layer and said secondgraphene layer are each permeable to Li+ charge carriers and eachindependently have an average thickness selected over the range of 5 nmto 100 nm.
 22. The electrochemical cell of claim 20, wherein said firstgraphene layer or said second graphene layer provide an elastic barrierwith said electrolyte capable of accommodating expansion or contractionof the volume of said silicon active material of said negative electrodeor said sulfur active material of said positive electrode.
 23. Theelectrochemical cell of claim 20, wherein said first graphene layer orsaid second graphene layer provide a chemical barrier capable ofpreventing transport of one or more reaction products from said siliconactive material of said negative electrode or said sulfur activematerial of said positive electrode and said electrolyte.
 24. Theelectrochemical cell of claim 23, wherein said second graphene layerprevents transport of polysulfides generated at said sulfur activematerials of said positive electrode to said electrolyte.
 25. Theelectrochemical cell of claim 1, wherein said electrolyte is a liquidphase electrolyte, gel electrolyte or a solid phase electrolyte havingan ionic conductivity for said charge carriers greater than or equal to1.5 S cm⁻¹.
 26. The electrochemical cell of claim 1, comprising asecondary electrochemical cell.
 27. The electrochemical cell of claim 1,comprising a lithium ion battery.
 28. The electrochemical cell of claim1, having a specific energy greater than or equal to about 387.5 Whkg⁻¹, or a standard cell voltage equal to or greater than 1.35 V or acycle life equal to or greater than about 1000 cycles.
 29. Theelectrochemical cell of claim 1, wherein said first assembly of carbonnanotubes and said second assembly of carbon nanotubes are independentlyprovided on an external surface of said substrate, or on one or moreintermediate structures provided between said first assembly of carbonnanotubes or said second assembly of carbon nanotubes and said externalsurface of said substrate.
 30. The electrochemical cell of claim 1,wherein said one or more first strips and said one or more second stripsarranged in the nested geometry.
 31. The electrochemical cell of claim1, wherein said one or more first strips and said one or more secondstrips arranged in the interleaved geometry, wherein one of the positiveor negative electrodes comprises recessed features and the other of thepositive or negative electrodes comprises projecting features.